Condition detecting apparatus



June 17, 1958 B. H. PINCKAERS CONDITION DETECTING APPARATUS Filed Dec. 12, 1955 IN VEV TOR.

United St tes Patent f' w 2,839,691 CGNDITION DETECTING APPARATUS Balthasar Hubert Pinckaers, Hopkins, Minn., assignor to Minneapolis-lloneywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application December 12, 1955, Serial No. 552,462

8 Claims. (Cl. 250-414 This invention is concerned with a condition detecting apparatus and more particularly with an apparatus for detecting the presence or absence of flame at a fuel burner unit.

More particularly, the present invention is concerned with an electronic type of flame detector wherein a flame sensing element, such as a photocell, is'arranged to monitor the flame at a fuel burner unit and to control a relay in accordance with the presence or absence of flame. Such electronic flame detectors have the distinct advantage of being very fast acting. That is, the electronic flame detector is capable of detecting the absence of flame in a very short time, as short as two to four seconds, and thereby turn off a main fuel valve to prevent an undue amount of fuel being fed to the fire box which no longer sustains a flame.

However, in the electronic type of flame detector, various components of the flame detector, such as the electron discharge devices, can fail to provide an unsafe operation of the detector. If a component of the flame detector fails in such a manner to render the flame detector incapable of sensing the presence of flame, such a failure, while being undesirable, is a safe failure since the main fuel valve cannot be opened. However, if a component within the flame detector fails in a manner to render the flame detector incapable of sensing the absence of flame, such a failure is an unsafe failure.

In other words, the electronic flame detector may function to open the main valve when in fact flame is not present at the burner unit, thereby allowing unburned fuel to accumulate in the fire box.

It is therefore an object of the present invention to provide a condition detecting apparatus which normally cannot fail in an unsafe manner.

It is a further object of the present invention to provide a flame detector having an electron discharge device whose state of conduction is controlled in accordance with the presence or absence of flame, and'having a pair of output relays and a cycling switch which are so interconnected so as to render one of the relays continuously energized to a moderate degree and the second degree energized only in the presence of flame, provided there are no component failures within the flame detector.

It is a further object of the present invention to provide a flame detector having flame sensing means and having a pair of electron discharge devices, one of which biases the other to cut off in the absence of flame, having a cycling switch which periodically renders the other elec tron discharge device conductive to a moderate degree independent of the presence of flame, and having a first and a second relay with coils connected inthe output circuit of the other discharge device, with the first relay energized to assume a marginalor mid-position upon moderate energization thereof and the second relay being energized only in the presence of flame, with cycling switch means alternately connecting the first and second relay to the output of the other electron discharge device I and at the same time alternately rendering the flame sensing means inoperative and then operative to sense flame.

These and other objects of the present invention will be apparent to those skilled in the art upon reference to the following specification, claims and drawings, of which: I

Figure l-is a schematic representation of the present invention associated with a fuel burner unit, and

Figure 2 is a bar graph showing the manner in which the motor-driven timer switches of Figure 1 are controlled.

Referring specifically to Figure l,'the reference numeral 10 generally designates a fuel burner unit having a pilot burner 11, a main burner 12, a pilot valve 13, a main valve 14, and an ignitiontransformer 15 associated in igniting relation to the pilot burner 11.

The flame at both the pilot burner 11 and the main burner 12 is monitored by a flame sensing means in the form of the photoelectric cell 16 of the photoemissive type having a cathode 58 and an anode 59. Photoelectric cell 16 is connected to an electronic flame detector 22 having a first electron discharge device 17 and a second electron discharge device 18.

Operation of the fuel burner 10 is controlled by means of a thermostat 19 which is arranged to be positioned in the space to be heated by the burner unit 10. The thermostat 19 functions upon a need for operation of the burner unit 10 to energize a main control relay 20 through a circuit which includes a safety switch 21.

, The electronic flame detector 22 includes a transformer 23 having a primary winding 24'which is connected to an alternating current source of voltage whose input conductors are designated by numerals 102 and 109. The transformer 23 also has a first secondary winding 25 which supplies operating voltage to the electron discharge devices 17 and 18, and likewise has a second secondary winding 26 which supplies power to the filaments of the electron discharge devices 17 and 18, through conductors, not shown.

As will be described, electron discharge device 17 is 30 of relay 27 is biased, by means not shown, to engage contact 31 upon relay winding 28 being de-energized. Upon a moderate energization of relay winding 28, the switch blade 30 moves into engagement with a spring 33. Spring 33 is not attached to switch blade 30 but is positioned so as to abut the switch blade upon the switch blade 30 moving into the mid-position as shown in Figure 1. Upon energization of relay winding 28 to a substantially higher degree, the switch blade 30 moves into engagement with contact 32, against the biasing of spring 33. Relay 27 is what might be called a marginal relay in that it has three operative positions depending upon, in the first position, a de-energized condition of relay winding 28, in the second position, a moderate energization or energization of relay winding 28 to a degree somewhat less than full energization, and, in the third position, a higher degree, or a full energization of winding 28.

A second delay relay 34 has a winding 35, shunted by a capacitor 36, which is also connected in the output circuit of electron discharge device 18. Relay .34 includes switch blades 37 and 38. Switch blades 37 and 38 are normally biased, by means not shown, to disengage contacts 39 and 40 respectively when relay winding 35 is de-energized. Upon energization of relay winding 35 the switch blades 37 and 38 move into engagement with contacts 39 and 40, respectively.

A motor-driven timer, designated by the reference numeral 41, and enclosed within a broken line, is also connected as a portion of the electronic flame detector 22. Motor-driven timer 41 includes an actuator 42 and four switch contacts designated S1 through S4, which are controlled in a cyclic manner by energization of the actuator 42. Actuator 42 is continuously energized from power line conductors 102 and 109.

Referring to Figure 2, a bar graph shows the switches S1 through S4 controlled in a cyclic manner so long as the actuator 42 remains energized. This cyclic manner of actuation of the switches causes the switches 51 and S2 to be closed for a first portion of the cycle of operation and to be opened for the remaining portion of the cycle of operation of the motor-driven timer 41. Likewise, the graph shows that the switches S3 and S4 are maintained open for this first portion of the cycle and are closed for the remaining portion of the cycle.

Operation of the apparatus of Figure 1 can best be explained by first considering the manner in which the electron discharge device 17 is connected in controlling relation to the electron discharge device 18. Electron discharge device 17 has a cathode 43, a control electrode 44, and an anode 45. The cathode 43 is connected by means of conductors 46, 47 and 48 to a lower terminal of the transformer secondary 25. The anode 45 of discharge device 17 is connected through a plate load impedance in the form of a potentiometer 49 to the tap 50 of transformer secondary 25.

The control electrode 44- of discharge device 17 is con nected to the cathode 43 through a filter network including the parallel connected capacitor 51 and resistor 52. With such a connection of the control electrode to the cathode, the discharge device 17 is normally conductive to cause a voltage of the polarity indicated in Figure 1 to exist across the resistance element 53 of the potentiometer 49.

However, a further circuit exists between cathode 43 and control electrode 44 when the switch S4 of the motor-driven timer 41 is closed. The apparatus of Figure 1 is shown with the motordriven timer 41 in the first portion of its cycle of operation wherein switch S4 is open. However, it will now be assumed that switch S4 is closed. A circuit can now be traced from the tap b of secondary winding 25 through the lower portion of secondary winding 25, conductors 48, 47, 46 and 54, photocell 16, conductor 53, switch S4, conductor 56, and a capacitor 57 to the tap 59 of secondary 25. In this above traced circuit it can be seen that since current can flow only from the cathode 58 to the anode 59 of photocell 16, the capacitor 57 will be charged to the polarity indicated. This charge will be distributed through a resistor 60 to the capacitor 51 and will charge capacitor 51 to the polarity indicated in Figure 1. This tends to bias electron discharge device 17 substantially to cutolf and the voltage which is normally present across the resistance element 53 of potentiometer 49 will no longer be present.

It will be remembered that the above traced circuit whereby capacitor 57 is charged exists only when the photocell 16 senses a flame at the burner unit and when switch S4 is closed. In the absence of such a flame this circuit, while being completed by switch S4 during the first part of the cycle of operation of the cycling switch 41, does not cause capacitor 57 to be charged and the voltage across the resistance element 53 of potentiometer 49 remains as shown in Figure 1.

Referring now to electron discharge device 13, this device includes a cathode 61, a control electrode 62, and an anode 63. The cathode 61 is connected by means of a conductor 64 to the tap 50 of secondary winding 25. The control electrode 62 is connected by means of a conductor 65 to the anode 45 of discharge device 17. In other words, the control electrode 62 and cathode 61 are connected across the resistance element 53 A of potentiometer 49, and this connection is in such a manner so as to bias discharge device 18 substantially to cutoff when a voltage exists across resistance element 53 due to current conduction of discharge device 17.

Electron discharge device 18 is in eifect a current conducting device which is controlled by the bias voltage applied between the control electrode 62 and the cathode 61. The current flow circuit, or anode to cathode circuit, for discharge device 18 is controlled by the switches 52 and S3 of the motor-driven timer 41. In Figure l, the motor-driven timer .-1 is shown in the posi tion where S2 is closed and S3 is open. In this con dition, the current flow circuit for the discharge device 18 can be traced from the tap 56 of secondary winding tnrough conductor 64, cathode 61 and anode 63, conductor 76, switch S2, conductor 71, winding 28, and conductors 72 and 77 to the upper terminal of transformer secondary 25. This current flow is a pulsating direct current and charges capacitor 2d which shunts winding 23. It will be noted that in this above traced circuit current can flow only while the switch S2 is closed. Referring to Figure 2, switch S2 is closed for only the first portion of the cycle of the motor-driven timer 41. The function of capacitor 29 is to store electrical energy while switch S2 is closed and dissipate this electrical energy through the winding 28 to maintain the winding energized for the period of time for which the switch S2 is open.

The second current flow circuit for discharge device 135 exists when switch S2 is open and switch S3 is closed. This circuit can be traced from the tap S6 of secondary through conductor 64, cathode 61 and anode 63, conductor 76, conductor 73, switch S3,,conductor 74, relay winding 35 shunted by capacitor 36, and conductors 75, 76 and 77 to the upper terminal of transformer secondary 25. In this circuit it can be seen that current can flow only during the remaining portion of the cycle of motor-driven timer 41 during which switch S3 is closed and switch S2 is open. Here again, the capacitor 36 functions to store electrical energy while current flows in this last traced circuit and to dissipate the electrical energy through the winding 35 while switch S3 is in the open condition.

Referring now to the function of the switch means of: relay 27, it can be seen that the movable switch blade is connected by means of conductor 78 to the lefthand terminal of winding of relay 34. Both of the contacts 31 and 32 which cooperate with switch blade 30 are connected by means of conductor and 76 to the right-hand terminal of the winding 35 of relay 34. From this it can be seen that when switch blade 31) is either in a position to engage contact 31 or to engage contact 32 the winding 35 is shorted and cannot be energized. As has been mentioned, these two positions of the switch blade 39 correspond to a state of complete de-energization of relay winding 28 and to a state of full energization of relay winding 28. When relay winding 28 is energized to an intermediate extent, the switch blade 30 assumes the position shown in Figure 1 wherein it moves to engage spring 33 and is then restrained from further movement.

Referring now to the function of motor-driven timer switch S1, this switch shunts a portion of the resistance element 53 of potentiometer 49. This can'be seen by tracing a circuit from the potentiometer tap 80 through conductor 81, switch S1, and conductors 82 and 64 to the left-hand terminal of the resistance element 53. The function of motor-driven timer switch S1 is to reduce the bias voltage applied to the control electrode 62 of discharge device 18 to allow discharge device 18 to be conductive to a moderate extent. In other Words, when discharge device 17 is conductive in response to the absence of flame at the burner unit 10, the discharge device 18 will be moderately conductive when the switch S1 is closed andwill be substantially cut off when the switch S1 is open.

At the sarnetime that switch S1 is closed, the switch S2 is closed and the current flow circuit for discharge device 18, which is moderately/conductive, includes the winding 28 of relay 27. This causes switch blade 30 to assume the intermediate position wherein it has engaged spring 33 and is restrained from further movement by the biasing action of this spring, as shown in Figure 1. During the next half cycle of timer 41 wherein the switches S1 and S2 are open, the switches S4 and S3 are closed. If flame is not present at the burner unit 10, discharge device 17 remains conductive and discharge device 18 is rendered nonconductive. Therefore, even though the switch S3 is closed connecting winding 35 of relay 34 into the currentflow circuit of discharge device 18, this relay winding is not energized.

If flame is present at the burner unit at this time the photoelectric cell 16 will be rendered conductive and will charge capacitor 57 torender discharge device 17 nonconductive. Voltage then no'longer exists across the resistance element 53 and discharge device 18 is rendered fully conductive. With discharge device 18 fully conductive and with switch S3 closed, relay winding 35 is energized to cause switch blades 37 and 38 to move into engagement wtih contacts 39 and 40, respectively.

In this manner, the photoelectric cell 16 is cyclically connected in controlling relation to the electron discharge device 17. If no flame is present at the burner unit 10, the discharge device 18 is rendered cyclically moderately conductive and then cut on, During the moderate conduction portion of the cycle the winding 28 is energized to maintain the switch blade in the intermediate position. During the cutoii portion of the cycle the wnding 35 is connected to the discharge device 18 and remains de-energized in response to the absence of flame at the burner unit 10. 1

Considering now the operation'when a flame is present at the burner unit 10, the photoelectric cell 16 is rendered conductive to bias discharge device 17 nonconductive during that portion of the cycle when switch S4 is closed. When switch S4 is closed, switch S3 is also closed. Since discharge device 17 is'nonconductive, there will not be a voltage present across resistance element 53 and discharge device 18 will therefore be fully conductive. With switch S3 closed, relay winding 35 is connected in circuit with discharge device 18 and is energized to cause switch blades 37 and 38 to engage contacts 39 and 40 respectively.

The remaining portion of the cycle of timer 41, the switch S4 is open and photocell 16 is rendered inoperative to sense flame at the burner unit 10. Discharge device 17 therefore becomes conductive to cause a voltage to exist across resistance element 53. However, at this time switch S1 is closed to shunt'a portion of resistance element 53. This causes discharge device to be rendered moderately conductive since the voltage developed across resistance element 53 as shunted is not sufficient to render discharge device 18 totally nonconductive. At this time switch S2 is closed and winding 28 is connected to be energized by the moderately conductive discharge device 18.

From this it can be seen that proper operation of the electron discharge devices 17 and 18 and their associated components is checked once every half'cycle of the motordriven timer 41. This is the half cycle such as shown in Figure 1 wherein the photocell 16 is rendered inoperative to sense the presence or absence of flame at the the discharge device 18 during this of the timer 41, winding 28 is energized to a degree in accordance with the biasing of discharge device 18. This maintains the switch blade 30 in the intermediate position shown in Figure 1. I

On the'succeeding half cycle of operation of the timer 41, the photocellv 16 is connected in controlling relation to the discharge device 17 and the relay 34 is connected in circuit with the discharge device 18. If a flame is present at the fuel burner 10, the relay 34 is energized.

If during the described checking half cycle of the motor-driven timer 41 the relay 28 is energized to a higher than moderate degree, such as would occur if the discharge device 17 were inoperative, the switch blade 30 will move into engagement with contact 32 to thereby shunt the winding 35 of relay 34 and prevent its energization on the next half cycle of the timer 41. If the discharge device 18 is nonconductive during this checking half cycle of the motor-driven timer 41, the switch blade 30 engages contact 31 and likewise shunts the winding 35 of relay 34 to prevent its energization. Failure of other components .of the electronic flame detector 22 can be considered to show thatthe operation of switch blade 30 will be effective to shunt the winding 35 of relay 34 and prevent energization of this Winding if a failure occurs in the electronic flame detector.

Referringinow to the complete operation of the apparatus as disclosed in Figure 1, upon a need for operation of the burner unit 10, the thermostat 19 closes its associated circuit to complete an energizing circuit for main relay 20. This energizing circuit can be traced from the upper terminal of the secondary of transformer through thermostat 1'9, winding 91,conductor '92, heating element 93 of safety switch 21, conductor 94, switch 95 of safety switch 21, and conductor 96 to lower terminal of the secondary of transformer '90.

Safety switch 21 is the type wherein a predetermined time period of energization ofthe heater 93 causes a bimetal 97 to warp to the right out from under the switch 95 and cause the switch to open. Upon cooling of the bimetal 97 a manual reset button 98 can be depressed to reset the safety switch to the position shown.

Energization of winding 91 of relay 20 causes switch blade 100 to move into engagement with contact 101. This completes an energizing circuit for the ignition transformer 25 and for the pilot valve 13. This circuit can be traced from the power line conductor 102 through conductor 103, contact 101 and switch blade 100, conductor 104, conductor 105, conductors 106 and 107 to parallel connected pilot valve 113 and ignition transformer 15, and conductor 108 to the other power line conductor 109. A flame is normally established at the pilot burner 11 at this time.

Establishment of flame at the pilot burner 11 is sensed by the photoelectric cell 16, in the manner above described, and relay 34 is. energized to move switch blades 37 and 38 into engagement with contacts 39 and 40, respectively. 1

Switch blade 37, in engaging contact 39, places a short circuit around the heating element 93 of the safety switch 21 to thereby prevent actuation of the safety switch contacts 95 to their open position.

Engagement of switch blade 38 with contact 40 completes an energizing circuit for the main valve 14 associated with the main burner 12. This energizing circuit can be traced from the power line conductor 102, conduct-or 103, contact 101 and switch blade 100, conductor 104, contact 40 and switch blade 38, conductor 110,- main valve 14, and conductors 111 and 108 to the other power line conductor 109. A flame is normally now established at the main burner 12.

Photoelectric cell 16 is efiective during that half cycle of the',motor-driven timer 41 during which switch S4 is closed to continuously monitor the flame at the pilot 75 burner 11 and the main burner 12, It flame should fail,

portion of the cycle a the flame relay 34 is de-energized, as above described, and movable switch blades 37 and 38 disengage contacts 39 and .40, respectively. This again places the heater93 of safety switch 21 in series with the winding 91 of relay 20. Likewise, the main valve 14 is tie-energized to stop the supply of fuel to the main burner 12. After a predetermined length of time, the safety switch contacts 95 are actuated to their open position and relay 2% is thereby d e-energized to cause switch blade 190 to disengage contact ltil. This de-energizes the pilot valve 13 and the ignition transformer 15.

Opening of the safety switch contacts 95 also de-energizes the safety switch heater 9 3, and a predetermined time there-after the bimetal 97 will cool and the safety switch contacts 95 can be reset by manually depressing the actuator 98. An attemptcan then be made to reignite, provided the thermostat 19 is still calling for operation of the burner unit 10.

From the above it can be seen that an improved flame detection device has been provided wherein a means such as the motor driven timer and its associated contacts provide for checking proper operation of the flame detector to insure that a component failure within the flame detector will not allow false indication of flame at the burner unit being monitored. Other modifications of the present invention will be apparent to those skilled in the art and it is intended that the scope of the presentinvention be limited solely to thescope of the appended claims.

ll claim as my invention:

1. Flame detector comprising: an electron discharge device having a cathode, an anode, and a control electrode; flame sensing means connected to said cathode and control electrode and arranged to bias said discharge device substantially to cut oil when said flame sensing means does not sense flame; a first relay having a coil, a second relay having a coil; cycling switch means having a first and second switch both of which are closed during a first portion of a cycle and open the remainder of the cycle, and having a third switch which is open during said first portion of the cycle and closed said remainder of the cycle; means controlled by said first switch arranged to control the bias of said discharge device to render said discharge device conductive when said first switch is closed during said first portion of the cycle even though said flame sensing means does not sense flame, means controlled by said second switch connecting the coil of said first relay to said anode and cathode to be energized during said first portion of the cycle; and means controlled by said third switch connecting the coil of said second relay to said anode and cathode to be energized during said remainder of the cycle only when said flame sensing means senses flame.

2. Flame detector comprising: a first electron discharge device whose state of conduction is controlled in accordance with the presence or absence of flame; said first electron discharge device being rendered nonconductive when a flame is sensed; impedance means connected in circuit with said first electron discharge device to thereby have a voltage present across said first impedance when said first discharge device is conductive; a second discharge device having a cathode, an anode, and a con trol electrode; circuit means connecting said impedance from said control electrode to said cathode so that the voltage across said impedance renders said second discharge device nonconductive; a flrst and a second relay both of which include a relay coil; cycling switch means having a first and a second switch both of which are closed during a first portion of the cycle and open during the remainder of the cycle, and having a third switch which is open during said first portion of the cycle and closed said remaining portion of the cycle; means controlled by said first switch arranged to shunt a portion of said impedance when first switch is closed to thereby reduce the bias voltage applied to said second discharge device and render said second discharge device conductive even though a flame is not being sensed, means controlled by said second switch connecting the coil of said first relay to said anode and cathode of said second discharge device to thereby energize the coil of said first relay during said first portion of the cycle; and means controlled by said third switch connecting the coil of said second relay to said anode and cathode to thereby energize the coil of said second relay during said remainder of the cycle only when a flame is sensed.

3. A flame detector comprising: controllable current conducting means, means including a flame responsive means arranged to control the state of conduction of said current conducting means to render said current conducting means substantially nonconductive in the abof flame and substantially conductive in the prescnce of flame; switching means including an actuator and a first, second and third switch, said actuator controlling said first, second and third switches in a cyclic manner to maintain said first and second switches closed for a first portion of the cycle and open during the remaining portion of the cycle, said actuator maintaining said third switch open during said first portion of the cycle and closed during said second portion of the cycle; first control means having a current controlled actuator; second control means having a current controlled actuator; means including said first switch controlling the state of conduction of saidv current conducting device to render said device conductive so long as said first switch is closed even though a flame is not sensed; means controlled by said second switch connecting said first control means actuator in series with said current conducting device to energize said actuator when said first'and said second switches are closed; and means controlled by said third switch connecting said second control means actuator in circuit with said current conducting device to energize said second control means actuator when said third switch means is closed and when a flame is sensed.

4. A flame detector comprising: an electron discharge device having a cathode, an anode, and a control eleo trode; means including flame sensing means connected to said cathode and control electrode and arranged to bias said discharge device substantially to cutofl when said flame sensing means does not sense flame; a first relay having a coil and switch means controlled thereby, a second relay having a coil and switch means controlled thereby; cycling switch means having a first, a second and a third switch, said first and second switches being maintained in a closed condition during a first portion of the cycle of the cycling switch means and open during the remaining portion of the cycle of the c cling switch means, said third switch being maintained open during said first portion of the cycle and being maintained closed during the remainder of the cycle; means con trolled by said first switch arranged in controlling rela tion to said first discharge device to render said first discharge device conductive when said first switch is closed during said first portion of the cycle even though said flame sensing means does not sense flame; means controlled by said second s vitch connecting the coil said first relay to said anode and cathode to thereby cause said first relay coil to be energized during said first portion of the cycle; means controlled by third switch connecting said second relay coil to said anode and cathode to thereby energize said second relay coil during said remainder of the cycle only when said flame sensing means senses flame; circuit means controlled by said first relay switch means arranged to short circuit said second relay coil upon said first relay coil being deenergized; and circuit means controlled by sail seconfi relay switch means when said second relay coil is energized.

5. A flame detector comprising: an electron discharge device having an anode, a cathode, and a control electrode; flame sensing means, means controlled by said flame sensing means connected to said control electrode and arranged to bias said discharge device substantially to cutoff when said flame sensing means senses the absence of flame and to allow substantial conduction in the presence of flame; cycling switch means having four switches, a first and second of which are closed during a first portion of the cycle and opened during the remainder of the cycle, and a third and fourth of which are opened during said first portion and closed said remainder of the cycle; a first relay having a coil and switch means, said switch means having a first operative condition when said first relay coil is de-energized, a second operative condition when said first relay coil is energized to a limited extent, and a third operative condition when said first relay coil is energized to a greater extent; a first capacitor, means connecting said first capacitor to shunt said first relay coil; a second relay having a coil and switch means, a second capacitor, means connecting said second capacitor to shunt said second relay coil; means controlled by said first cycling means switch arranged to reduce the cutoff bias of said discharge device to allow limited conduction of said discharge device during said first portion of the cycle; means controlled by said second cycling means switch arranged to connect said first relay coil to said anode and cathode to provide limited energization of said first relay coil during said first portion of the cycle; means controlled by said third cycling means switch arranged to connect said second relay coil to said anode and cathode to provide energization of said second relay coil during said remainder of said cycle only when said flame sensing means senses flame; means controlled by said fourth cycling means switch arranged to render said flame sensing means inoperative to sense flame during said first portion of the cycle; means controlled by said first relay switch means to shunt said second relay coil when said first relay switch means assumes said first or third operative condition; and means controlled by said second relay switch means when said second relay coil is energized.

6. A flame detector for use with a fuel burner unit comprising a first electron discharge device having a control electrode, and an anode and cathode; a second electron discharge device having a control electrode, and an anode and cathode; a plate load impedance, means connecting said plate load impedance to said anode and cathode of said first electron discharge device, means connecting said plate load impedance to said control electrode and cathode of second electron discharge device so that the potential across said plate load impedance controls the state of conduction of said second electron discharge device; cycling switch means having four switches, a first and second of which are closed during a first portion of the cycle and opened during the remaining portion of the cycle, and a third and fourth of which are open during said first portion of the cycle and closed during said remaining portion of the cycle; flame sensing means arranged to be positioned to sense the flame at the fuel burner unit, means including said fourth switch connecting said flame sensing means to the control electrode of said first electron discharge device, said-connection being completed during said remaining portion of the cycle during which said fourth switch is closed, said flame sensing means rendering said first electron discharge device nonconductive in the presence of flame at the fuel burner unit; means controlled by said first switch of said cycling switch means arranged to. shunt a portion of said plate load impedance and thereby vary the bias on the control electrode of said second discharge device during the first portion of said cycle when said first switch is closed; a first and a second relay each having switch means and each having a relay coil; means controlled by said second switch of said cycling switch means connecting said first relay coil to said anode and cathode of said second electron discharge device during said first portion of said cycle when said second switch is closed; circuit means controlled by said third switch of said cycling switch means connecting the coil of said second relay to the anode and cathode of said second discharge device during the remaining portion of said cycle when said third switch is closed; circuit means controlled by the switch means of said first relay when said first relay is de-energized or completely energized to shunt the coil of said second relay, and circuit means controlled by the switch means of said second relay when said second relay is energized.

7. Condition detecting apparatus comprising: an electron discharge device having a cathode, an anode, and a control electrode; condition sensing means connected to said cathode and control electrode and arranged to bias said discharge device substantially to cutotf when said condition sensing means does not sense a condition to be detected; a first relay having a coil, a second relay having a coil; cycling switch means having a first and second switch both of which are closed during a first portion of a cycle and open the remainder of the cycle, and having a third switch which is open during said first portion of the cycle and closed said remainder of the cycle; means controlled by said first switch arranged to control the bias of said discharge device to render said discharge device conductive when said first switch is closed during said first portion of the cycle even though said condition sensing means does not sense the condition, means controlled by said second switch connecting the coil of said first relay to said anode and cathode to be energized during said first portion of the cycle; and means controlled by said third switch connecting the coil of said second relay to said anode and cathode to be energized during said remainder of the cycle only when said condition sensing means senses the condition.

8. A condition detector comprising: controllable current conducting means, means including a condition responsive means arranged to control the state of conduction of said current conducting means to render said current conducting means substantially nonconductive in the absence of the condition and substantially conductive in the presence of the condition; switching means including an actuator and a first, second and third switch, said actuator controlling said first, second and third switches in a cyclic manner to maintain said first and second switches closed for a first portion of the cycle and open during the remaining portion of the cycle, said actuator maintaining said third switch open during said first portion of the cycle and closed during said second portion of the cycle; first control means having a current controlled actuator; second control means having a current controlled actuator; means including said first switch controlling the state of conduction of said current conducting device to render said device conductive so long as said first switch is closed even though the condition is not sensed; means controlled by said second switch connecting said first control means actuator in series with said current conducting device to energize said actuator when said first and said second switches are closed; and means controlled by said third switch connecting said second control means actuator in circuit with said current conducting device to energize said second control means actuator when said third switch means is closed and when the condition is sensed. 

